If two bureaucrats both use Special:UserRights to adjust an accounts' flags, their changes do not interleave, but just over-write.
For example, if User:Foo is currently +rollbacker, and two bureaucrats each at the same time try to adjust their rights, bureaucrat A by setting Foo +sysop and bureaucrat B setting Foo +bot, User:Foo should now be +rollbacker +sysop +bot, but instead with either be +rollbacker +sysop or +rollbacker +bot depending on whether A or B hit the database first.
This is akin to edit-conflicts. And yes, a very minor enhancement. :-)